Ultra Power Saving Mode

I just got my S6 after having an S5 and I don't know if this is the same for everyone but I wanted to ask.

I used to be able to get 12 days out of my phone on UPS on s5 but the s6 only goes for 3.5 days? I obviously don't need it that long but why such a huge difference? Is it battery size? More processing power eating the battery even in UPS or something else?